Light emitting diodes and laser diodes are well known solid state lighting elements capable of generating light upon application of a sufficient voltage. Light emitting diodes and laser diodes may be generally referred to as light emitting devices (“LEDs”). Light emitting devices generally include a p-n junction formed in an epitaxial layer grown on a substrate such as sapphire, silicon, silicon carbide, gallium arsenide and the like. The wavelength distribution of the light generated by the LED generally depends on the material from which the p-n junction is fabricated and the structure of the thin epitaxial layers that make up the active region of the device
Typically, an LED chip includes a substrate, an n-type epitaxial region formed on the substrate and a p-type epitaxial region formed on the n-type epitaxial region (or vice-versa). In order to facilitate the application of a voltage to the device, an anode ohmic contact may be formed on a p-type region of the device (typically, an exposed p-type epitaxial layer) and a cathode ohmic contact may be formed on an n-type region of the device (such as the substrate or an exposed n-type epitaxial layer).
LEDs may be used in lighting/illumination applications, for example, as a replacement for conventional incandescent and/or fluorescent lighting. As such, it is often desirable to provide a lighting source that generates white light having a relatively high color rendering index (CRI), so that objects illuminated by the lighting may appear more natural. The color rendering index of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. The color quality scale (CQS) is another objective measure for assessing the quality of light, and similarly ranges from essentially zero to nearly 100.
In addition, the chromaticity of a particular light source may be referred to as the “color point” of the source. For a white light source, the chromaticity may be referred to as the “white point” of the source. The white point of a white light source may fall along a locus of chromaticity points corresponding to the color of light emitted by a black-body radiator heated to a given temperature. Accordingly, a white point may be identified by a correlated color temperature (CCT) of the light source, which is the temperature at which the heated black-body radiator matches the color or hue of the white light source. White light typically has a CCT of between about 4000 and 8000 K. White light with a CCT of 4000 has a yellowish color. White light with a CCT of 8000 K is more bluish in color, and may be referred to as “cool white”. “Warm white” may be used to describe white light with a CCT of between about 2600 K and 6000 K, which is more reddish in color.
In order to produce white light, multiple LEDs emitting light of different colors of light may be used. The light emitted by the LEDs may be combined to produce a desired intensity and/or color of white light. For example, when red-, green- and blue-emitting LEDs are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the component red, green and blue sources. However, in LED lamps including red, green, and blue LEDs, the spectral power distributions of the component LEDs may be relatively narrow (e.g., about 10-30 nm full width at half maximum (FWHM)). While it may be possible to achieve fairly high luminous efficacy and/or color rendering with such lamps, wavelength ranges may exist in which it may be difficult to obtain high efficiency (e.g., approximately 550 nm).
In addition, the light from a single-color LED may be converted to white light by surrounding the LED with a wavelength conversion material, such as phosphor particles. The term “phosphor” may be used herein to refer to any materials that absorb light at one wavelength and re-emit light at a different wavelength, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Accordingly, the term “phosphor” may be used herein to refer to materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors absorb light having shorter wavelengths and re-emit light having longer wavelengths. As such, some or all of the light emitted by the LED at a first wavelength may be absorbed by the phosphor particles, which may responsively emit light at a second wavelength. For example, a single blue emitting LED may be surrounded with a yellow phosphor, such as cerium-doped yttrium aluminum garnet (YAG). The resulting light, which is a combination of blue light and yellow light, may appear white to an observer.
However, light generated from a phosphor-based solid state light source may have a relatively low CRI. In addition, while light generated by such an arrangement may appear white, objects illuminated by such light may not appear to have natural coloring due to the limited spectrum of the light. For example, as the light from a blue LED covered by a yellow phosphor may have little energy in the red portion of the visible spectrum, red colors in an object may not be well-illuminated. As a result, the object may appear to have an unnatural coloring when viewed under such a light source. Accordingly, it is known to include some red-emitting phosphor particles to improve the color rendering properties of the light, i.e., to make the light appear more “warm”. However, over time, the red-emitting phosphor particles may be subject to greater degradation than the yellow-emitting phosphor particles, which may decrease the useful lifetime of the light source.
Accordingly, there is a continued need for improved LED lighting sources for general illumination.